2 Review of Structure Building a Structural Model of Erwin Chargaff studied the nitrogen bases in the of many species Noticed that amount of adenine equals thymine and amount of cytosine equals guanine Chargaff s rule Rosalind Franklin & Maurice Wilkins used X - ray crystallography to uncover the double helix of Sugar phosphate backbone on outside Watson & Crick purine paired with a pyrimidine (purine to purine would make helix too wide) Side groups on each nitrogen base forms a hydrogen bond between them 2

3 Watson & Cricks Hypothesis of Replication Each strand serves as a template for the new strand Origin of Replication 1. Replication begins at specific sites where the two parental strands separate & form replication bubbles 2. Bubbles expand laterally, as replication proceeds in both directions 3. Replication bubbles fuse & synthesis of daughter strands is complete Antiparallel Elongation Enzyme polymerase adds new nucleotides to the template strands Two strands of run antiparallel (opposite directions) polyermase III adds nucleotides in the 5 to 3 direction Leading strand new strand made in 5 to 3 direction Lagging strand strand produce by nucleotides being added in the direction away from the replication fork ; synthesis occurs with a series of segments called Okazaki fragments Ligase enzyme that joins the sugar phosphate backbones of Okazaki fragments to form new strand 3

4 Priming Synthesis polymerases cannot initiate synthesis of a polynucleotide (can only add nucleotides to the 3 end) Primer initial nucleotide chain; short & consist of either or RNA ; initiate replication Primase enzyme that can start a stretch of RNA from scratch 1. Primase joins RNA nucleotide to primer 2. polymerase III adds nucleotides to 3 end of primer 3. Continues adding nucleotides 4. polymerase I replaces the RNA nucleotide with versions Proteins For Leading & Lagging Strands PROTEIN FUNCTION Helicase Unwinds double helix at replication fork Single-strand binding protein Stabilizes single-stranded until used as a template Topiosomerase Corrects overwinding ahead of replication fork Summary of replication Protein Function for Leading Strand Function for Lagging Strand Primase pol III pol I Ligase Synthesizes a single RNA primer at 5 end of leading strand Continuously synthesizes the leading strand, adding on to primer Removes primer from 5 end of leading strand & replaces it with Joins the 3 end of that replaces the primer to rest of leading strand Synthesizes an RNA primer at the 5 end of each Okazaki fragment Elongates each Okazaki fragment, adding on to its primer Removes the primer from the 5 end of each fragment & replaces it with, adding on to 3 end of adjacent fragment Joins Okazaki fragments 1. Helicase unwinds double helix 2. Single-stranded binding proteins stabilize template strands 3. Leading strand is synthesized continuously in 5-3 direction by pol III 4. Primase begins synthesis fo RNA primer for 5 th Okazaki fragment 5. pol III completes synthesis of fourth fragment; when it reaches RNA primer on third fragment it will break off & move to replication fork, and add nucleotides to 3 end of fifth fragment 6. pol I removes primer from 5 end of second fragment, replacing it with nucleotides that it adds one by one to 3 end of third fragment. Replacement of last RNA nucleotide with leaves the sugar-phosphate backbone with a free 3 end 7. ligase binds 3 end of the second fragment to the 5 end of the first fragment 4

5 Proofreading & Repairing polymerases proofread nucleotides & immediately replace any incorrect pairing Some mismatched nucleotides evade proofreading or occur after synthesis is complete damaged Mismatch repair cells use special enzymes to fix incorrect nucleotide pairs 130 repairing enzymes identified in humans to date Nucleotide Excision Repair Thymine dimer type of damage caused by UV radiation Buckling interrupts replication Replicating Ends of Molecules Since only adds to 3 end, no way to complete 5 ends Even if Okazaki fragment is started with a an RNA primer, it can not be replaced with when removed Results in shorter molecules Problem exists only in eukaryotes due to linear Prokaryote is circular Telomeres nucleotide sequences at ends of molecules Consist of multiple repetitions of a short sequence TTAGGG human telomere Telomeres prevent the erosion of genes near the ends of molecules Somatic cells of older individuals have shorter telomeres related to aging? In germ cells the erosion of telomeres can lead to loss of essential genes Telomerase enzyme that catalyzes the lengthening of telomeres; restores the original length 5

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1 copyright cmassengale 2 History of DNA Early scientists thought protein was the cell s hereditary material because it was more complex than DNA Proteins were composed of 20 different amino acids in long

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OpenStax-CNX module: m45475 1 DNA Replication * OpenStax This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License 4.0 By the end of this section, you will be able